1. Field of the Invention
The present invention relates to an organic light-emitting diode (OLED) display and a method of fabricating the same. More particularly, the present invention relates to a top emission OLED display using an auxiliary electrode for preventing or reducing a voltage drop of an upper electrode such that the top emission OLED display may be large-sized and a method of fabricating the same.
2. Description of the Related Art
A conventional top emission active matrix organic light-emitting diode (AMOLED) display uses a transparent cathode electrode in order to emit light toward a sealing substrate.
In general, a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO) is mainly used as the transparent cathode electrode. However, in order to function as the cathode electrode, a metal material having a low work function is thinly deposited on one side of the transparent conductive layer that comes in contact with an organic layer to form a semitransparent metal layer, and then the ITO or IZO is thickly deposited on the semitransparent metal layer.
In the above process, since the ITO or IZO layer is formed after the organic layer is formed, the ITO or IZO layer should be formed by a low temperature deposition method in order to minimize damage of the organic layer due to heat or plasma. When the low temperature deposition method is used, however, the quality of the ITO or IZO layer is deteriorated and its specific resistance is increased.
When the specific resistance of the cathode electrode is increased, voltage differences are generated between near regions and far regions from a portion where a power source is input due to a voltage drop depending on positions of the pixels rather than uniformly applying a cathode voltage to all pixels. As a result, non-uniformity of brightness and image characteristics may be generated and power consumption may increase.
Also, due to the voltage drop, it is difficult to apply to a middle-sized or large-sized top emission AMOLED display.
In order to solve the above problem, Shoji Terada et al. introduced a method of forming an auxiliary electrode for preventing a voltage drop of an upper electrode on a pixel-defining layer 285 in “54.5L: Late-News Paper: A 24-inch AM-OLED Display with XGA Resolution by Novel Seamless Tiling Technology,” SID Symposium Digest 34, 1463 (2003).
A conventional top emission OLED display will now be described with reference to the attached drawings.
FIG. 1 is a partial cross-sectional view of a conventional top emission OLED display, showing only a portion corresponding to a thin film transistor, a pixel electrode, and a capacitor.
Referring to FIG. 1, a buffer layer 110 is formed on an insulating substrate 100. An active layer 120 including source and drain regions 121 and 125 is formed on the buffer layer 110. A gate electrode 141 and a lower electrode 147 of a capacitor are formed on a gate-insulating layer 130. Formed on an interlayer insulating layer 150 are source and drain electrodes 161 and 165 connected to the source and drain regions 121 and 125 through contact holes 151 and 155, respectively, and an upper electrode 167 of the capacitor connected to one of the source and drain electrodes 161 and 165, for example, the source electrode 161.
A passivation layer 170 is formed on the entire surface of the insulating substrate 100. A lower electrode 180, i.e., a pixel electrode, is formed on the passivation layer 170 as an anode electrode of an electroluminescent (EL) device connected to one of the source and drain electrodes 161 and 165, for example, the drain electrode 165, through a via hole 175. A pixel defining layer 185 having an opening 189 which exposes a portion of the lower electrode 180 is formed. An organic layer 190 is formed on the lower electrode 180 in the opening 189. Then, an auxiliary electrode 193 for preventing a voltage drop of an upper electrode is formed on the pixel defining layer 185, and an upper electrode 195 serving as a cathode electrode is formed on the entire surface of the insulating substrate 100.
However, according to the above method, in a process of forming the auxiliary electrode 193, when a semitransparent metal layer used as the auxiliary electrode 193 is deposited and patterned on the pixel defining layer 185, the organic layer 190 may be damaged. Also, a masking process should be added to form the auxiliary electrode 193, which leads to the complication of the process.